Thin films of organic dielectric materials, such as polyimides, have found widespread use in the electronics industry in a variety of applications. One area in which thin films of polyimide and organic dielectric materials have found extensive use is in the fabrication of packaging modules for carrying one or more integrated circuit ("IC") chips. A plurality of individual IC chips may be disposed upon a so-called multichip module comprising a substrate upon which the chips are mounted. The substrate will typically contain various signal, power and ground lines for connection to the IC chips mounted thereon. The substrate may also comprise passive components such as bypass capacitors and terminal resistors.
In order to include the necessary power, signal and ground lines, (and to include, if applicable, passive components), the substrates used in multichip modules generally comprise a plurality of metal layers separated by dielectric material. Initially, most multichip modules were made of co-fired ceramic materials. In the last several years, thin-film packaging technologies have become more significant for fabricating multichip modules, particularly so-called copper/polyimide based modules. Thin film IC packages of this type typically comprise alternating layers of thin films of patterned copper and polyimide materials, usually on a thick base support substrate. Most recently, multichip modules combining both ceramic and thin-film technologies have been developed. A general discussion of both types of technologies may be found in Chapters 7 and 9 of Microelectronics Packaging Handbook, R. Tummala, et al. (eds.), (1989, van Nostrand Reinhold, New York).
The term polyimide, although sometimes used in a context which suggests a particular compound, actually describes a class of ring-chain resin polymers containing imide fragments. Polyimides are particularly useful because they have high temperature tolerance, good mechanical and chemical properties, low dielectric constant, and are relatively easily formed into a film which is highly uniform in thickness. While polyimides are the most commonly used class of dielectric materials used in thin film packages, other organic materials may also be used. Other organic materials that have been used or proposed for use in multichip modules include bensocyclobutenes, polyphenylquinoxalines, and various oligomers. The curing and polymerization processes for such materials are similar to the process used with polyimides.
Typically, a layer of the desired organic dielectric material (or its precursor) is applied to the base substrate (upon which other layers may have already been formed) in the liquid state. The layer is then hardened (polymerized or immidized) by using a curing process. The curing process normally involves two aspects, removal of the solvent carrier and imidization or cross-linking of the remaining material. The most common method of curing polyimides, and other thin film organic dielectric layers, is by heating them at an elevated temperature for a relatively long time. Provided that the curing temperature is sufficiently high, the heat curing process fulfills both aspects of the process, i.e., the solvent base is removed by volatilization at high temperature, and the immidization process also occurs at high temperature. As is well-known in the art, for temperature-induced immidization to occur, the curing temperature must be above the glass transition temperature of the polyimide material being used. As is also well-known in the art, the glass transition temperature increases with the degree of curing. For example, the initial glass transition temperature of a solvent-free uncured polyimide film may be approximately 150.degree. C., while the glass transition temperature of the same film rises to above 300.degree. C. after the film is cured.
Thus, a typical polyimide, as applied, may consist of a polyamic acid or polyamic ester dissolved in a solvent of N-Methyl-2-pyrrolidone. The latter has a boiling point of about 202.degree. C. After solvent removal, the uncured polymeric material has a glass transition temperature of approximately 150.degree.-250.degree. C. However, for the reasons described below, the polymerization reaction must, typically be conducted at above 350.degree. C., which is the glass transition temperature of the cured film.
As noted, the heat-curing technique generally in use in connection with thin films of organic dielectric materials used in multichip modules involves heating the layer to a high temperature, typically about 400.degree. C., for a relatively long period of time (one to four hours per layer). This process limits the maximum number of organic film layers that can be used due to the degradation of the polymer/metal interfaces caused by the high temperature curing.
Another problem with high temperature curing of polyimide and other organic layers arises from the mismatch between the thermal coefficients of expansion (TCEs) of the organic layers and the interposed metal layers and the base substrate. If the system is cured at a temperature greater than the final glass transition temperature of the organic film, the stress at the interface between the film and the metal layer will be zero at the final glass transition temperature, i.e., the film will be fully relaxed at the glass transition temperature. As the layers are cooled down below this temperature, stress builds up due to the mismatch between the TCEs of the layers, causing bowing of the substrate. This effect limits the size of the multichip module and choice of materials used to create it.
In addition, if the polyimide or other organic material layer is used in connection with active devices, such devices may be damaged or destroyed by the high temperatures used in the curing process.
U.S. Pat. No. 5,024,969, entitled Hybrid Circuit Structure Fabrication Methods Using High Energy Electron Beam Curing, issued Jun. 18, 1991 to Reche, discloses a technique for curing polyimide layers in a multichip module using e-beam irradiation of the uncured layer. Under the preferred process described in the '969 patent, the polyimide film is cured at room temperature. In an alternate embodiment, the curing may be conducted at the anticipated operating temperature of the multichip module, so long as the curing temperature is lower than the glass transition temperature of the uncured polyimide layer. However, the '969 patent recognizes that the quality of a polyimide layer cured at room temperature is poor due to inferior adhesion to the metal layers. It is also believed that films cured in this manner have poor chemical resistance. To solve the problem of poor adhesion, the '969 patent suggests a post-cure bake of the overall multichip module at a temperature less than the transition temperature of the polyimide. It is assumed that the problem of poor adhesion arises from the failure, when curing at room temperature, to drive off the solvent used to carry the polyimide, and that the baking step is used to complete the solvent removal. While the process described in the '969 patent is, in some respects, an improvement over the temperature curing technique, it is believed that the quality of the resulting films is still low, and that the post-cure bake step adds unnecessary complexity and added fabrication time.